For many years it has been an aim of scientists in the field of specifically targeted drug therapy to use monoclonal antibodies (MAbs) for the specific delivery of toxic agents to human cancers. Conjugates of tumor-associated MAbs and suitable toxic agents have been developed, but have had mixed success in the therapy of cancer in humans. The toxic agent is most commonly a chemotherapeutic drug, although particle-emitting radionuclides, or bacterial or plant toxins, have also been conjugated to MAbs, especially for the therapy of cancer (Sharkey and Goldenberg, CA Cancer J Clin. 2006 July-August; 56(4):226-243).
The advantages of using MAb-chemotherapeutic drug conjugates are that (a) the chemotherapeutic drug itself is structurally well defined; (b) the chemotherapeutic drug is linked to the MAb protein using very well-defined conjugation chemistries, often at specific sites remote from the MAbs' antigen binding regions; (c) MAb-chemotherapeutic drug conjugates can be made more reproducibly and usually with less immunogenicity than chemical conjugates involving MAbs and bacterial or plant toxins, and as such are more amenable to commercial development and regulatory approval; and (d) the MAb-chemotherapeutic drug conjugates are orders of magnitude less toxic systemically than radionuclide MAb conjugates, particularly to the radiation-sensitive bone marrow.
Camptothecin (CPT) and its derivatives are a class of potent antitumor agents. Irinotecan (also referred to as CPT-11) and topotecan are CPT analogs that are approved cancer therapeutics (Iyer and Ratain, Cancer Chemother. Phamacol. 42: S31-S43 (1998)). CPTs act by inhibiting topoisomerase I enzyme by stabilizing topoisomerase I-DNA complex (Liu, et al. in The Camptothecins: Unfolding Their Anticancer Potential, Liehr J. G., Giovanella, B. C. and Verschraegen (eds), NY Acad Sci., NY 922:1-10 (2000)). CPTs present specific issues in the preparation of conjugates. One issue is the insolubility of most CPT derivatives in aqueous buffers. Second, CPTs provide specific challenges for structural modification for conjugating to macromolecules. For instance, CPT itself contains only a tertiary hydroxyl group in ring-E. The hydroxyl functional group in the case of CPT must be coupled to a linker suitable for subsequent protein conjugation; and in potent CPT derivatives, such as SN-38, the active metabolite of the chemotherapeutic CPT-11, and other C-10-hydroxyl-containing derivatives such as topotecan and 10-hydroxy-CPT, the presence of a phenolic hydroxyl at the C-10 position complicates the necessary C-20-hydroxyl derivatization. Third, the lability under physiological conditions of the δ-lactone moiety of the E-ring of camptothecins results in greatly reduced antitumor potency. Therefore, the conjugation protocol is performed such that it is carried out at a pH of 7 or lower to avoid the lactone ring opening. However, conjugation of a bifunctional CPT possessing an amine-reactive group such as an active ester would typically require a pH of 8 or greater. Fourth, an intracellularly-cleavable moiety preferably is incorporated in the linker/spacer connecting the CPTs and the antibodies or other binding moieties.
A need exists for more effective methods of preparing and administering antibody-CPT conjugates, such as antibody-SN-38 conjugates. Preferably, the methods comprise optimized dosing and administration schedules that maximize efficacy and minimize toxicity of the antibody-CPT conjugates for therapeutic use in human patients.